Structure of end states for a Haldane Spin Chain

Inelastic neutron scattering was used to probe edge states in a quantum spin liquid. The experiment was performed on finite length antiferromagnetic spin-1 chains in Y_2BaNi_{1-x}Mg_xO_5. At finite fields, there is a Zeeman resonance below the Haldane gap. The wave vector dependence of its intensity provides direct evidence for staggered magnetization at chain ends, which decays exponentially towards the bulk (xi = 8(1) at T=0.1K). Continuum contributions to the chain end spectrum indicate inter-chain-segment interactions. We also observe a finite size blue shift of the Haldane gap.